Infrared radiation (IR) detectors are typically manufactured in the form of a two-dimensional juxtaposition (for example, in an array) of an assembly of elementary microbolometers arranged at the surface of a support substrate, each microdetector being intended to form an image point. Each microdetector comprises a membrane suspended above the substrate and electrically connected thereto by means of long narrow beams (or “arms”) embedded in electrically-conductive pillars. The assembly is placed in a tight enclosure, for example, a package under very low pressure, to suppress the thermal conductance of the surrounding gas.
Each membrane heats up by absorbing the incident radiation originating from the observed thermal scene, which is transmitted and focused by an adequate optical system at the level of the focal plane having the membranes arranged thereon. The membrane comprises, in particular, a layer of a “transducer” material having an electric property, the resistivity in the case of microbolometers, which strongly varies when the temperature changes, for example generating a current variation under a constant voltage biasing, that is, an electric signal, proportional to the incident radiation flow.
Conventional methods of manufacturing detectors of this type comprise steps directly carried out at the surface of a substrate comprising a plurality of electronic circuits or “read-out integrated circuits” or “ROICs”, in so-called “monolithic” fashion. This term designates a continuous sequence of operations on the same substrate, after the integrated circuit manufacturing process, usually based on silicon. Bolometric microdetector manufacturing steps are generally similar to collective manufacturing techniques usual in microelectronics, usually concerning from a few tens to a few hundreds of array detectors arranged on a same substrate.
During the manufacturing steps, the components implementing the bolometric functions of optical absorption, optical-thermal transduction, and thermal resistance, are formed at the surface of a so-called “sacrificial” layer, in that the layer, simply intended to form a construction base, is removed at the end of the process by an adequate method which does not attack the other detector parts, and particularly the components formed thereon. Typically, a polyimide layer is used, which layer is eventually removed by combustion in an oxygen plasma. As a variation, the sacrificial layer is a silicon oxide layer (generally designated by “SiO” eventually removed by hydrofluoric vapor phase etching (HFv). After the removal of the sacrificial layer, the bolometric membranes remain suspended above the substrate, with no other contact or fastening than their holding arms embedded in the pillars.
The most common manufacturing method for forming suspended membranes is called “above IC” or “MEMS-on-top”. According to this method, the microdetectors are directly constructed at the surface of the substrate comprising the read-out circuits, due to specific methods. Particularly, the sacrificial layer is of organic nature—generally polyimide—and the transducer material most often is an oxide having a semiconductor character (VOx, NiOx) or amorphous silicon (a-Si). Conventionally, for the usual “far” infrared detection (LWIR), a beam splitter is also formed between the absorbing membrane and a reflector arranged at the substrate surface, to provide an absorption maximum for the detector in the vicinity of 10 micrometers. Thus, to connect and hold the membrane at an adapted distance from the reflector, to form said beam splitter in vacuum, electric pillars with a large aspect ratio, usually rather complex and having a non-negligible bulk, should be formed through a thick temporary (sacrificial) polyimide layer having a thickness in the range from 2 to 2.5 micrometers.
The dielectric or resistive layers which form the membrane “skeleton” are conventionally made of silicon oxide (SiO) or of silicon nitride (generically noted SiN), or also directly of semiconductor amorphous silicon according, for example, to U.S. Pat. No. 5,912,464. Such materials can be deposited at relatively low temperature and are inert towards the method of removing the organic sacrificial layer under an oxygen plasma. Such an “above IC” manufacturing process is typically formed of some ten photolithographic “levels”, that is, according to a relatively complex and expensive process.
More recently, a new type of membrane manufacturing has been provided, which comprises integrating the microbolometers in the so-called “back-end of line” layers (or “BEOLs”), in the same way as the components generally achieving the MEMS functions. This acronym designates the steps of manufacturing all the metal interconnects at relatively low temperature, characteristic of the end of standard microelectronic manufacturing processes. Such an approach, called “MEMS-in-CMOS”, aims at using certain BEOL layouts, mature on an industrial level, to integrate part of the microbolometers components. In particular, the metallized vertical interconnection vias between successive BEOL metal levels, for example obtained according to the “damascene” method, advantageously form the microdetector pillars. Further, “IMDs” (Inter-Metal-Dielectrics), in particular made of SiO, a standard material in microelectronics, may advantageously be used as sacrificial layers for the construction of membrane structures. In this type of integration, the last photolithographic levels for the read-out circuit manufacturing are also used to directly form the pillars supporting the membranes. A few lithographic levels in the series of levels necessary to manufacture microbolometers are thus spared, which results in a significant saving on manufacturing costs. However, the removal of the SiO sacrificial layer of the “MEMS-in-CMOS” manufacturing is in this case only feasible by means of vapor-phase hydrofluoric acid (HFv). Accordingly, all the materials forming the microbolometers should imperatively be inert with respect to this very chemically aggressive method.
Such a “MEMS-in-CMOS” approach, applied to the case of microbolometers, has been described in document US 2014/319350, which details the integration in the CMOS stack of a SiO sacrificial layer and of a barrier layer enabling to contain the HFv etching, as well as the forming of the microbolometer “pillars” by using the last structure of connection between standard metal levels (metallized vias) of the CMOS assembly. This document more particularly describes a microbolometer construction based on amorphous silicon which uses the teachings of document U.S. Pat. No. 5,912,464 for the membrane architecture. A structure compatible with the HFv etching and formed by means of five photolithographic levels only is thus obtained, which provides a very significant gain as compared with the much more complex process of the state of the art.
While the “MEMS-in-CMOS” technique enables to simplify the manufacturing, it however suffers from limitations which penalize the performance of the microbolometers thus constructed.
In particular, the architecture provided according to this technique imposes a sharing of the space available between metallized areas intended, in particular, to absorb the incident radiation, and areas only occupied by the transducer material (amorphous silicon). The fraction of the surface area occupied by the metal conditions the optical-thermal transduction function (optical absorption efficiency ε of the membrane), while the remaining surface area fraction is dedicated to the thermoelectric transduction function in the transducer material. Such a limitation of the volume of material implied in the electric conduction (as compared with the total volume of amorphous silicon present in the structure) generates a decrease in the number of charge carriers N implemented in the conduction. This necessarily results in a substantial increase in the low-frequency noise (“B1/f ”) in accordance with Hooge's relation, which penalizes the signal-to-noise ratio (“SNR”) of the detector.
To better understand this issue, reference is made to FIGS. 1 to 3, illustrating an elementary resistive bolometric microdetector 10 (or “microbolometer”) of the state of the art for infrared detection. Bolometer 10 comprises a thin membrane 12 absorbing the incident radiation, suspended above a substrate—support 14 via two conductive anchoring pillars 16 to which it is attached by two holding and thermally-insulating arms 18. In the illustrated example, membrane 12 comprises two metal elements 20, 22 having an IR absorbing and biasing electrode function, and an amorphous silicon layer 24 covering each of the two electrodes 12, 14 and filling space 18 therebetween. Layer 24 has a function of transduction of the heating caused by the absorption of the radiation by electrodes 20, 22 into an electric resistance variation. In this structure, the transducer material is thus only made of amorphous silicon, which has the advantage of being inert with respect to the sacrificial layer releasing process based on vapor-phase hydrofluoric acid.
Response (V/K) of a microbolometer of electric resistance Rb biased under a constant voltage Vpol expresses the output signal variation ∂S in relation with a scene temperature variation ∂θsc according to general relation:
                    ℛ        =                                            ∂              S                                      ∂                              θ                sc                                              ∝                                    Vpol              Rb                        ⁢            A            *            ɛ            *            TCR            *                          R              th                        *                                          ∂                                  θ                  ⁡                                      (                                          θ                      sc                                        )                                                                              ∂                                  θ                  sc                                                                                        (        1        )            where:                A is the total area of the sensitive elementary point (detector pixel),        ε is the general optical absorption efficiency of the bolometer,        TCR is the variation coefficient of the bolometer resistance according to the membrane temperature;        Rth is the thermal resistance between the membrane and the substrate (that is, the holding arms), and        {circle around (×)}(θsc) is the radiation flow emitted by the scene at temperature θsc.        
As previously mentioned, optical absorption efficiency ε is linked to the fraction of the surface area of each membrane occupied by the metal deposited for this purpose.
The electric resistance of a microdetector Rb can be expressed according to resistivity ρ of the transducer material, for example, according to relation:
                    Rb        =                  ρ          *                      L                          W              *              e                                                          (        2        )            where L, W and e respectively are the length, the width, and the thickness of the volume of transducer material (assumed to have or taken down to a parallelepipedal shape) conducting the electric current.
In the example of membrane of FIG. 1, these dimensions are substantially those of the area separating electrodes 20, 22, for example corresponding to a physical interrupt (or groove) formed in an initially continuous layer of metal to form the electrodes (which are in this example typically called “coplanar” since they are arranged at the same level).
The combination of relations (1) and (2) thus enables to specify the response of a microdetector according to the dimensional parameters of the involved resistor Rb.
The current noise power of a resistor biased under a voltage Vpol can be expressed by the quadratic sum of the so-called 1/f low-frequency noise (Ib1/f) and of a frequency-independent component called “white noise” (Ibb). The ultimate noise linked to thermal fluctuations can be neglected as compared with these first-order contributors.
Noise power Ib1/f2 varies according to the inverse of number N of charge carriers contained in the volume concerned by the current lines, according to Hooge's relation:
                              I                      b            ⁢                                                  ⁢                          1              /              f                                2                =                                            α              H                        N                    *                                    (                              Vpol                Rb                            )                        2                    *                      ln            ⁡                          (              BPCL              )                                                          (        3        )            where αH is the “Hooge parameter” and “BPCL” is the frequency bandwidth of the read-out circuit. Each material is characterized by a reference ratio
            α      H        n    ,where n is the charge carrier volume density; this ratio further depends on temperature. Thus, for a resistor Rb of known dimensions, the real ratio
      α    H    Nof the considering element is simply calculated from dimensional parameters W, L, e according to relation:
                                          α            H                    N                =                                            α              H                        n                    *                      (                          1                              W                *                L                *                e                                      )                                              (        4        )            
White noise power Ibb2 only depends on temperature and on the resistance of the considered element according to relation:
                              I          bb          2                =                                            4              *              k              *              T                        Rb                    *                      (            BPCL            )                                              (        5        )            where k designates Boltzmann' s constant and T designates temperature.
A microdetector provided with a portion of transducer material characterized by its ratio
      α    H    Nand its resistance Rb, defined from known dimensions W, L, and e, thus exhibits a total noise Ib which can be expressed according to relation:
                              I          b                =                              [                                                                                4                    *                    k                    *                    T                                    Rb                                *                                  (                  BPCL                  )                                            +                                                                    α                    H                                    N                                *                                                      (                                          Vpol                      Rb                                        )                                    2                                *                                  ln                  ⁡                                      (                    BPCL                    )                                                                        ]                                              (        6        )            
The microdetector signal-to-noise ratio (SNR) can be calculated by the ratio of the response (1) to the noise (6) by taking into account the elements defined by the read-out circuit (Vpol, BPCL) and the dimensional parameters of the resistor of each microdetector (W, L, e) which enable to express bolometric resistance Rb and the number of charge carriers N. Ratio SNR can thus be expressed according to relation:
                    SNR        ∝                  (                      ℛ                          I              b                                )                                    (        7        )            
To give a simplified, though representative, illustration of a pixel of very small dimensions such as it is currently useful, or even necessary, to industrially provide, the case of an elementary bolometric detector having a 12×12-μm2 surface area is considered.
To define the sharing of this available surface between a metallized fraction 20, 22 necessary for the absorption of the thermal radiation, and an electrically-active fraction 26 intended for thermoelectric transduction, a groove of (electric) length L etched in a metal layer across the entire (electric) width W=12 μm of this element may be conveniently defined. The ratio of surface area occupied by the metal is then equal to (12-L)/12, while the length and width of the resistor respectively are L and W=12 μm. Equations (2), (4), and (7) result in bolometric resistance (Rb), in ratio
            α      H        N    ,and then finally in the SNR according to design length L.
Thus, neglecting, for simplification, the spaces consumed to form the separations between adjacent membranes and the sub-structures such as pillars, holding arms, and other various necessary spaces, an area of 12*12 μm2 comprising two rectangular metallized portions separated by a groove of length L, which is also the length of an amorphous silicon transducer volume of width W=12 μm, of indicative resistivity 100 Ω.cm, of typical thickness e=200 nm, and of ratio
            α      H        n    =            6.7      ⁢      E        -          28      ⁢                          ⁢              m        3            is considered in the following as schematized in FIG. 1, resistance Rb and the SNR have a variation according to distance L illustrated in FIG. 4.
To provide an optimal optical absorption efficiency ε, it is necessary to set the length of the non-metallized areas to values at most in the range from 2 to 3 μm. This spacing (length L) is further necessary to maintain resistance Rb of the elementary bolometer within a range of values compatible with an adequate use of the read-out circuit, in terms of biasing Vpol, of read integration time, and of useful dynamics (with no saturation) of the output amplifier. This condition is typically satisfied when bolometric resistance Rb is in the order of or smaller than approximately 1 MOhm. For more explanations in relation with these elements, reference will for example be made to “Uncooled amorphous silicon technology enhancement for 25 μm pixel pitch achievement”; E. Mottin et al, Infrared Technology and Application XXVIII, SPIE, vol. 4820E.
Thus, with a length L set to 2 μm to guarantee a state-of-the-art absorption efficiency and an acceptable resistance, the SNR of this microdetector based on amorphous silicon will be limited to approximately 60% of its maximum value corresponding to large lengths L (not including absorption losses). Such a limitation is linked to the increase of the low-frequency noise at low values of L.
It is thus difficult, or even impossible to obtain an acceptable tradeoff between the absorption efficiency and the SNR for sensitive pixels of small dimensions formed according to this simplified assembly, particularly for pitches smaller than 20 μm. The use of low-noise transducer materials at low frequency, such as semiconductor metal oxides (VOx, TiOx, NiOx, for example—generic denomination “MOx” will be used hereafter), would in principle enable to overcome this limitation. However, the use of such materials in the stack of the state of the art is not possible, since they would be rapidly removed or at least strongly degraded under the effect of the very aggressive HFv releasing chemistry.
There thus is a need, at least in the context of microbolometer assemblies partly integrated to a CMOS process, that is, where the sacrificial material is made of SiO or of any related material conventional in microelectronics, for high-performance devices and for methods of manufacturing the same compatible with the design of retinas of very small pitch, typically below 20 μm.